E:.xcess 210Pb and 210Po

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1 E:.xcess 210Pb and 210Po in sédiment from the Western North Pacific Koh Harada Yoko Shibamoto Introduction Particles in most surficial marine sédiments are mixed mostly by the biological activities on the sea floors and the mixing is called as bioturbation. Knowledge ofthe mechanisms and rates ofthe biotur bation is critical to understand rates of the recycling and burial of organic matter, biogenic silica, carbonate and manganèse (Bemer, 1980; Emerson, 1985; Aller, 1990). To détermine the rate of the bioturbation, excess 2l0Pb, which was produced ultimately from 226Ra and scavenged by settling or resuspended particles, has been widely used (e.g., Nozaki et al, 1977; Peng et al, 1979; DeMaster and Cochran, 1982). The western North Pacific has relatively high biological activity in sur face water compared to the eastern North Pacific. This means rain rate of organic carbon to the sea floor in the western North Pacific is high and high bioturbation rate is expected. To clarify the rate, 2inPb profile in the sédiment from the western North Pacific was determined. Activity ratio of 2l0Po to 21cPb in particulate matter in water column is generally over unity (Harada and Tsunogai, 1986). This suggests that significant amount of 210Po reaches to the sea floor and some part of the 2H,Po exists in the sédiment even though half-life of 2l0Po is short, 138 days. To test this hypothesis, 21()Po in the sédiment was also determined.

224... Environmental Changes and Radioactive Tracers 1Methods Coring and sample treatment on

of Japanese JGOFS activity, is located at M on, 155 E in the western North Pacifie.

KNOT twice in cruises of RJV Hakurei-Maru #2 in October 1998 and 180Ê -15DÊ -l 20Ê -9DÊ.

2 Environmental Changes and Radioactive Tracers 1Methods Coring and sample treatment on board of the ship Station KNOT (Figure 1), which is a station for time series observations of Japanese JGOFS activity, is located at M on, 155 E in the western North Pacifie. Sediment core samples were collected at Stn. KNOT twice in cruises of RJV Hakurei-Maru #2 in October 1998 and 180Ê -15DÊ -l 20Ê -9DÊ.SlJË 60 Ê 3DÊ DÊ -3DÊ -60Ê ' 2 5 1' I:IIDE I :U:,E t tas 1 sa! I!CS'..~ D l'do HJa 1Figure 1 Map of the northwestern North Pacifie showing a core location.

K. HARADA, Y. SHIBAMOTO - Exœss 2l0pb in sediment 'rom the Western North Pacifie... 225 RN Mirai in November 1998 using with a multiple corer (Figure 2).

for the rest. The sliced samples were frozen and transferred to a land laboratory. In the 1aboratory, the samples were dried and powdered.

3 K. HARADA, Y. SHIBAMOTO - Exœss 2l0pb in sediment 'rom the Western North Pacifie RN Mirai in November 1998 using with a multiple corer (Figure 2). Immediately after recovery of the corer on the deck, the sediment core sampie was sliced by 3 mm thickness for the top ten sarnples, by 6 mm thickness for the next 10 samples and by 12 mm thickness for the rest. The sliced samples were frozen and transferred to a land laboratory. In the 1aboratory, the samples were dried and powdered. Water content and porosity were calculated from weight Joss after drying. 1Figure 2 Picture of the Multiple Corer in the RN Mirai cruise. Radiochemical analyses Radioactivities of 22fiRa and 2IOPb in the samples were determined by gamma spectrometry with a well-type intrinsic germanium detector. Efficiencies of the detector were calibrated by IAEA sediment standards. Activity of 210Po in the samples was determined by alpha spectrometry using 209PO as a chemical yield monitor (Harada and Tsunogai, 1985). Aliquot arnount of the sediment sampje was leached in 6 M Hel solution and polonium was collected in hydroxide precipitate

4 226 T Environmental Changes and Radioactive Tracers from the leachate. Activities of 209Po and 210Po were determined by silicon surface barrier detectors after spontaneous déposition of polonium onto a sil ver disk from 0. 1 M HCI that dissolved the precipitate. Results and discussion 226Ra and 210Pb in the sédiment at Stn. KNOT Vertical distributions of 226Ra and 210Pb in the two sédiment cores from Stn. KNOT were shown in Figure 3. Concentration of 2,0Pb decreased rapidly from surface down to 1 cm depth, and then decreased gradually down to 6 cm depth. Below the depth, the con centration was almost constant, whereas there was a peak at 8 cm depth in the 1998 sample core. Contrastively, concentration of 226Ra, which is a precursor of 2l0Pb, was almost constant in the cores, ranging between 3.5 to 8.0 dpm.g1. The vertical profiles of 226Ra and 2l0Pb showed that there was excess 210Pb over its precursor 226Ra from surface down to 6 cm depth. This excess 2l0Pb must be supported by an input from outside of the sédiment, which is an input of 210Pb from the water column by set tling particles. Fluxes of210pb from the bottom water to the sédiment Assuming a steady state condition, the 2l0Pb flux from the overlying water column to the sédiment can be calculated from inventories of 226Ra and 210Pb in the sédiment from following relation, F, = Mi l where À,j is a decay constant of nuclide i (day-1) and I; is an inven tory in the sédiment of the nuclide i (dpm.nr2).

5 K. Harada, Y. Shibamoto Excess 210Pb in sédiment from the Western North Pacific 227 dpm.g-' dpm.g-' y s ' 1 November ' J6Ra 15 2,0Pb i I Figure 3 Vertical distibutions of 226Ra and 210Pb in the sédiment core at Stn. KNOT. The flux of 210Pb was calculated as 81.9 and 64.2 dpm.m"2.day"' respectively from the October and November, profiles. This cal culated flux can be compared with the flux observed by a sédiment trap experiment. Unfortunately, 2,0Pb in the sédiment trap samples hâve not been determined so far, whereas the experiment at Stn. KNOT was carried out by the other group. Judging from previous papers, 2,0Pb flux observed by the sédiment trap in the North Pacific ranged from 20 to 100 dpm.m2.day_l. The calculated fluxes from the profiles in the sédiment seem to be comparable with the fluxes observed by the sédiment traps in the biologically productive area in the North Pacific. 210Po in the sédiment Using the sédiment samples collected in November 1998, 2I0Po was also determined, which is a daughter nuclide of 2l0Pb. The vertical

6 228' Environmental Changes and Radioactive Tracers profile of 2l0Po showed in Figure 4 with 2l0Pb. Excess 210Po over 2l0Pb seems to be exist in the top two sub-samples and between 7 and 10 cm depth. Generally speaking, the settling particles hâve high 2l0Po/210Pb ratio from 3 to 10 and this was explained by preferential scavenging of 210Po from seawater to particulate matter. Although there still exist a possibility of calibration error ofthe counter because the excess 210Po in the deeper samples is not explainable, the excess 210Po in the core top seems to be caused by higher particulate flux of 210Po from the water column than of 2,0Pb. The 210Po flux from the water column can be estimated from the inventory ofthe excess 210Po in the sédiment in the same method as 2l0Pb flux mentioned above. The flux was calculated as 590 dpm.m^.day"' showing the estimated 2iop0/2iopD rau-0 jn tne settiing particle was 7, which is comparable or a little higher than the observed one in the North Pacific. I Figure 4 Vertical distibutions of 2,0Pb and 2,0Po in the sédiment core collected in October. dpm.g-'

7 K. Harada, Y. Shibamoto Excess 2,0Pb in sédiment from the Western North Pacific T 229 Estimation of bioturbation rate from the vertical profiles of210pb and2wpo As shown above, excess 2l0Pb and 2,0Po were detected in the sub surface layers of the sédiment core from the western North Pacific. Even though the excesses were supplied from the overlying water column by the settling particles, there must be some mechanisms in which surface sédiment particles was mixed with sub-surface one since accumulation rate of pelagic sédiment is very slow, <1 cm.kyr1, compared to the time scale of mean life ofthe nuclides. The particle mixing by organisms on sea floor, bioturbation, seems to be prédominant although the mixing by bottom water current could be occur. If the nuclides is carried to the interior of the sédi ment only by the bioturbation, the concentration of the nuclides in the sédiment can be expressed by the following équation, da d2a 0dA ^ A = Da r + S -XA 2 dt B dz2 dz where A is a concentration of nuclide i in the sédiment, DB is a bio turbation rate constant, S is a sédiment accumulation rate and lj is a decay constant of nuclide i. Since the second term in the right side in the Eq.(2) is negligible, the steady state concentration of the nuclide i can be expressed as follows; -Ie* A = C-e^B The concentrations of 2l0Pb were plotted against depth semilogalismically (Figure 5). Judging from the linearity, the sédiments were separated into three layers, surface to 1 cm, 1 to 2.5 cm and 3 to 6 cm depth and the biotubation coefficient in the layers was estimated to be 0.04, 0.4, 0.06 cm2.yr', respectively. It is very curious that the second layer has the largest coefficient, however, the reason is still unknown. Using the excess 210Po profile, the coefficient was also calculated. The estimated value, 1.2 cm2.yr', is significantly larger than one obtained from the excess 2l0Pb. To clarify the différence between the values from 210Pb and 2,0Po, further study is needed although the âge dépendent mixing of deep-sea sédiments was reported (Smith et al, 1993).

8 230' Environmental Changes and Radioactive Tracers excess210pb and 210Po(dpm.g'1) : 1.2cm2yr1 r t t r T-r-r r i i i i i i ' i ' i ri r i E mtr 0.4cm2yr"1, Q. (D 0.04 cm2 yr rt -, 12-1 'i /y 0.07 crn yr ^ Pb (Oct.'98) 210Pb (Nov. "98) 21l>o (Oct/98) I Figure 5 Semi logarithmic plots of 2,0Pb and 2' Po in the sédiment and estimation of DB. Conclusion In the sédiment in the western North Pacific, 1. excess 2l0Pb and excess 2l0Po existed down to 6 and 1 cm depth, respectively, 2. particulate fluxes of 2l0Pb and 210Po from the overlying water were estimated as 70 and 590 dpm.g1, respectively, 3. bioturbation mixing coefficient in the sédiment was estimated to be from to 0.4 cm2.yr' and the highest value was observed in cm layer.

9 K. Harada, Y. Shibamoto Excess 2,0Pb in sédiment from the Western North Pacific 231 Aknowledgements The authors would like to thank scientists, technicians and crew members on IW Mirai for their help of sample collection. Bibliography Aller R. C, 1990 "Bioturbation and manganèse cycling in hemipelagic sédiments". In H. Charnock et al. (eds) The Deep Sea Bed: Its Physics, Chemistry and Biology, Cambridge.University Press: Berner R.A., 1980 Early Diagenesis: A Theoretical Approarch. Prinston, University Press. DeMaster D.J., CochranJ.K., 1984 Particle mixing rates in deep-sea sédiments determined from excess 2' Pb and 32Si profiles. Earth Planet. Sti. Lett., 61: 257. Emerson S. R., 1985 "Organic carbon préservation in marine sédiments". In: Sundquist E.T., Broecker W.S. (eds): The Carbon Cycle and Atmospheric CÛ2: Natural Variations Archean to Présent, AGU Geophys: Harada K., Tsunogai S., A practical method for the simultaneous détermination of 234Th, 226Ra, 210Pb and 2' Po in seawater. J. Oceanogr. Soc. Japan, 41: 98. Harada K., Tsunogai S., 1986 Fluxes of 234Th, 2, Po and 210Pb determined by sédiment trap experiments in pelagic océans. J. Oceanogr. Soc. Japan, 42: 192. Nozaki Y., Cochran J. K., Turekian K. K., Keller G., 1977 Radiocarbon and 21 Pb distribution in submersible-taken deep-sea cores from project FAMOUS. Earth Planet. Sci. Lett., 34: 167. Peng T. H., Broeker W.S., Berger W. H., 1979 Rates of benthic mixing in deep-sea sédiments as determined by radioactive tracers. Quat.Res., 11:141. Smith C. R., Pope R. P., DeMaster D.J., Magaard L., 1993 Age-dependent mixing of deep-sea sédiments. Geochim. Cosmochim. Acta, 57: 1473.

Fluxes of 234Th, 210Po and 210Pb Determined by Sediment Trap Experiments in Pelagic Oceans*

Journal of the Oceanographical Society of Japan Vol. 42, pp.192 to 200, 1986 Fluxes of 234Th, 210Po and 210Pb Determined by Sediment Trap Experiments in Pelagic Oceans* Koh Haradat and Shizuo Tsunogait